This scenario happens every so often. It is good to have a plan!
In many Wards, the operative for me has been to do the the Christmas Choir "Program" (the day when the choir sings the most songs) the week before Christmas. In my wards, I could basically plan on my choir members going off on vacations or visiting relatives for Christmas. Sometimes, it was my family that left. In any case, having the Ward Choir sing on the Sunday nearest Christmas was just not possible.
So, we decided to use a Christmas Carol-Sing - a program using scriptures and messages from the living prophets interspersed with carols and other hymns from the hymnal. The Narrator(s) read the script and the Congregation sang the songs. Everybody was happy. One year, we even had all of our college kids (home for the holidays) be the Narrators. They even sang a special musical number. (They were talented kids!)
Writing a Christmas Carol-Sing script is pretty straightforward.
1. Pray for Inspiration and do your research. Perhaps start with finding a Christmas message from the First Presidency. Extract the best quotes. (See one of my scripts as an example)
2. Plan which scriptures to use. Perhaps some scriptures are referenced in the First Presidency Message or in the hymns.
3. Plan which hymns and carols to use. Perhaps one text contains the over-arching message or theme you want to highlight. Good themes are "Joy to the World" or "Come, O Thou King of Kings." These evoke very visual or emotional reactions from your audience. It gets them "on board" beginning with the title of the program.
4. Think of who will be around to direct the singing, play the organ or read the narrations. Play to their strengths. Give the more descriptive passages to the person who can deliver those parts the best, be aware of the limitations of your musicians, etc.
5. Time everything from line delivery to singing (how many verses?), to preludes, to movement to and from the podium. Make sure you stick within the time frame.
6. Do you need to leave time for a Bishop's Message? Put that into the plan.
7. Make your script legible and print enough copies.
8. Then, even if you have to be gone for Christmas, the program will not disappoint!